The present invention is directed generally to communication systems. More particularly, the invention relates to wavelength selection and receiving techniques for use in optical receivers and transmission systems.
The continued development of digital technology has provided electronic access to vast amounts of information. The increased access to information has driven demand for faster and higher capacity electronic information processing equipment (computers) and transmission networks and systems linking the processing equipment (telephone lines, cable television (CATV) systems, local, wide and metropolitan area networks (LAN, WAN, and MAN)).
In response to this demand, communications companies have turned to optical transmission systems to provide substantially larger transmission capacities than traditional electrical communication systems. Early optical transmission systems, known as space division multiplex (SDM) systems, transmitted one information signal using a single wavelength in single waveguide, i.e. fiber optic strand. Time division multiplexing (TDM) multiple low bit rate, information signals onto a single wavelength in a known sequence that can be separated upon receipt has further increased the transmission capacity of optical systems.
The continued growth in traditional voice, video, and data communications systems and the emergence of the Internet as a means for accessing data has further accelerated the demand for higher capacity transmission systems. Communications service providers, especially long distance telecommunications companies, have looked to wavelength division multiplexing (WDM) to further increase the capacity of their existing systems.
Additional transmission capacity is added to WDM systems by increasing the number of information carrying optical signal wavelengths, or signal channels, used in the system. Generally, unique optical transmitter/receiver pairs operated at fixed transmit/receive wavelengths are deployed to provide additional signal channels in WDM systems. The transmitters and receivers used in the WDM systems are generally the same in construction, except for the wavelength transmitted or received. Different wavelength optical sources or selective devices are provided in the transmitters and corresponding different optical filters or local oscillators are provided in the optical receivers to provide the different signal channels.
In optical systems, one of the more common techniques for selecting individual wavelength signal channelsinvolves the use of grating technology, usually fiber Bragg gratings (“FBG”). Fiber Bragg gratings have proven to be extremely useful wavelength selective devices, because the fiber gratings can be spliced directly into a transmission fiber and used to provide nearly distortion free separation and stabilization of optical signal wavelengths. Also, fiber Bragg gratings can be produced having well controlled reflectivities and reflective bandwidths. These attributes make Bragg gratings very well suited for use as optical filters in optical receivers and wavelength stabilizers in optical transmitters. See U.S. Pat. No. 5,077,816.
A current shortcoming of fiber Bragg gratings is the reflective wavelengths can only be efficiently tuned over a relatively narrow range, typically around 1 nm. It is therefore necessary to provide different fiber Bragg gratings for each different wavelength that must be separated or stabilized in the WDM system.
The need to use different Bragg gratings for each wavelength increases the complexity of manufacturing and maintenance of WDM systems. Whereas, a broadly tunable filter would streamline filter manufacturing by allowing the same device to be manufactured and then tuned to a desired operating wavelength when deployed in WDM systems. Tunable filters can also provide wavelength agility in optical transmitters and receivers, which allows flexible wavelength allocation and network planning and protection in WDM systems.
Another wavelength filtering technique employs Fabry-Perot (“F-P”) filters, which can be used to separate wavelengths from the WDM signal. Unlike Bragg gratings, Fabry-Perot filters can be tuned over a relatively wide wavelength range. However, narrow bandwidth F-P filters can introduce unacceptable levels of distortion into the filtered optical signals. As such, narrow bandwidth F-P filters havegenerally been limited to use in non-signal processing applications, such as in optical spectrum analyzers and other power measurement devices, and laboratory apparatuses and test systems.
The lack of a robust tunable wavelength selection technique constrains current WDM system designs and manufacturing capabilities. It will become increasingly necessary to provide tunable wavelength selection techniques to facilitate continued growth in WDM system capacity and capability. In view of these present constraints, there is a clear need for improved wavelength selection techniques and optical receivers and systems to facilitate the development of higher capacity, longer distance optical communication systems.